It could be shorting out, but most likely cause is an open circuit or faulty solder connection between the motherboard ATX power supply socket or a problem with the PSU's power wire plug that connects to it.
You may have to remove the motherboard so you can inspect the connection of the lugs on the soldered side of the motherboard, to see if one is loose and moving. If this is the case, you will need to purchase a soldering iron and solder and re-tin the connection.
What I described above is just a basic description, precautions have to be taken and if you intend to attempt this repair, please ask for further instructions before proceeding.
Doing this proceedure incorrectly can cause more damage to the connecter or motherboard.
Edited by MrBruce1959, 19 July 2010 - 07:30 PM.